1. Field of Invention
Composite fuels that can be converted to hydrogen include materials that endothermically release hydrogen, materials that exothermically release hydrogen, and additives to promote endothermic reaction and to inhibit exothermic reaction.
2. Description of the Related Art
Hydrogen is considered a future fuel for the globe. It contains high energy, enables high energy conversion efficiency, and is environmentally benign. Hydrogen enabled fuel cell power could be widely used for transportation, stationary, and portable power applications. Through a fuel cell device, hydrogen fuel electrochemically combines with oxygen from air to generate power with heat, and leaves water as the only exhaust. But hydrogen must be extracted or made from hydrogen-containing materials, such as water and hydrocarbons, or from synthesized chemicals like hydrides and alcohols. Issues from hydrogen production, storage and transportation have slowed down the realization of a hydrogen economy. To compete with existing hydrocarbon energy or batteries, these issues have to be resolved. One of the solutions is to develop technologies that enable hydrogen production at the point of applications in the quantity needed. With such a technology, there will be no needs to store and transport hydrogen.
The primary fuel selected to generate hydrogen plays a very important role in system energy efficiency and cost. U.S. Pat. No. 5,904,913 teaches a method to generate hydrogen from methanol. A mixture of methanol and water serves as the fuel to generate hydrogen in a steaming reformer in accordance with the following reaction:CH3OH(g)+H2O(g)→3H2(g)+CO2(g) ΔH°=+49 kJ/mol  (1)
Since this is an endothermic reaction, a large amount of heat energy is needed to maintain the reaction (over 300° C. even in the presence of catalysts). Furthermore, at such a high temperature methanol fuel also decomposes into carbon monoxide:CH3OH(g)→CO(g)+2H2(g) ΔH°=+96 kJ/mol  (2)This reaction takes more thermal energy than reforming reaction (1) and also releases poisonous carbon monoxide. The presence of carbon dioxide and carbon monoxide in the product gas stream will greatly reduce the value of this process for low temperature fuel cells, such as alkaline fuel cells (AFC) and polymer electrolyte membrane fuel cells (PEMFC). Several stages of treatment and separation of the product stream are needed before piping the hydrogen product into fuel cell devices. These treatments further reduce energy efficiency, add cost, and make the system very bulky and complex, which greatly limits its application. In addition of these problems, using methanol as the primary fuel also leads to the risk of exposure of methanol vapor at high temperature, creating safety concerns, especially for portable power applications.
U.S. Pat. No. 6,699,457 discloses a low temperature hydrogen production method using oxygenated hydrocarbon fuels, such as sugar and glucose. These fuels are reformed in liquid phase in the presence of catalysts, which helps to save some energy because there is no need to vaporize the fuel. However, a temperature of 300° C. for most of the fuels mentioned in the disclosure still must be maintained by an external heat source. Because of the slow kinetics of reforming these fuels, a large catalyst package is required, which limits application of the process to stationary fuel cell power. The presence of carbon dioxide in the product steam also requires separation for specific applications, such as alkaline fuel cell power generation. Because the process temperature is much higher than the boiling point of water, a high pressure vessel is needed to conduct liquid phase reforming, greatly reducing the gravimetric energy density of the system and creating safety concerns for portable power applications.
U.S. Pat. No. 6,534,033 discloses another way to generate hydrogen, using sodium borohydride at a lower temperature. The fuel used in this approach is a mixture of 20 wt % sodium borohydride and 3 wt % sodium hydroxide, with the rest water. When hydrogen is needed, the fuel mixture is pumped into a catalyst bed and sodium borohydride is hydrolyzed on the surface of promoting catalysts. The hydrolysis of sodium borohydride is represented as follows:BH4−(l)+2H2O(l)→BO2−(s)+4H2(g)ΔH°=−270 kJ/mol  (3)
If proper catalysts are used, this hydrolysis reaction will spontaneously take place. The fuel does not generate harmful vapors. However, this process generates a lot of waste heat and drives the temperature of the fuel processor up to 200° C., which causes rapid loss of water vapor and leaves a solid by-product (sodium borate) in the catalyst bed. A cooling system or extra energy is needed to remove this waste heat. Furthermore, sodium hydroxide in this fuel mixture serves as a stabilizer to prevent the hydrolysis of sodium borohydride solution during storage. In order to have adequate shelf storage time in practice, sodium hydroxide with a concentration of up to 40 wt % is needed. The extra alkali materials reduce the fuel energy density, increase fuel cost, and create environmental risk upon disposal of the fuel cartridge. Another problem with this fuel is its freezing point. The fuel storage container or cartridge can only be used at temperatures above 0° C. or even above 18° C. This problem limits its application to areas with warm climates. However, hydrogen enabled fuel cell power is often needed at temperatures of −20° C. or even lower.
The prior art is focused on methods to process a single primary fuel to generate hydrogen, but fails to make hydrogen generation simple and more energy efficient. There remains a need to develop fuels and corresponding processing methods to generate hydrogen on-demand at the point of application. A primary fuel for hydrogen generation should have the following: (a) the cost of fuel should be low; (b) the fuel should be very stable at a wide range of climatic conditions; (c) there should be no chemical decomposition or change of physical form during the storage time; (d) the fuel should require minimum energy to be processed or to achieve high-energy efficiency; (e) when the fuel is processed, there should be no harmful gases produced; (f) pure hydrogen should be produced in a single, simple step; and (g) the gravimetric and volumetric energy density of the fuel should be high.